Carbon Dioxide Dimer Radical Anion as Surface Intermediate of Photoinduced CO₂ Reduction at Aqueous Cu and CdSe Nanoparticle Catalysts by Rapid-Scan FT-IR Spectroscopy

Monitoring of visible light sensitized reduction of CO₂ at Cu nanoparticles in aqueous solution by rapid-scan ATR FT-IR spectroscopy on the time scale of seconds allowed structural identification of a one-electron intermediate and demonstrated its kinetic relevancy for the first time. Isotopic labeling (¹²C: 1632, 1358, 1346 cm^-1; ¹³C: 1588, 1326, 1316 cm^-1) revealed a species of carbon dioxide dimer radical anion structure, most likely bound to the catalyst surface through carbon. Intermediacy of Cu-C(═O)OCO₂^- surface species is in agreement with a recently proposed mechanism for electrocatalytic CO₂ reduction at Cu metal nanoparticles based on Tafel slope analysis. Spontaneous decrease of the intermediate after termination of the photosensitization pulse (Sn porphyrin excited at 405 nm) was accompanied by the growth of HCO₃^-. CO was produced as well, but sensitive detection required photolysis for tens of minutes. A direct kinetic link between a C₂O₄^- surface intermediate and the CO product was also demonstrated for photocatalyzed CO₂ reduction at aqueous CdSe nanoparticles, where first order growth of a Cd-C(═O)OCO₂^- species was accompanied by rise of CO (monitored by a fast Ni complex trap) and HCO₃^- showing a distinct induction period. The detection of the one-electron surface intermediate and confirmation of its catalytic relevancy was enabled by the delivery of electrons one-by-one by the photosensitization method. The observation of carbon dioxide dimer radical anion points to approaches for rate enhancements of heterogeneous CO₂ reduction by creating catalytic environments that favor formation of this intermediate.